Model Organisms in Drug Discovery

96 DROSOPHILA AS A TOOL FOR DRUG DISCOVERY
processes give stereotypic structural and physiological outputs. Thus,
Drosophila could be thought of as an alternative assay platform containing
most of the assay components and by which compound activity is assessed.
This argument follows the same underlining logic used for developing
successful robust non-mammalian assays, such as the amphibian melanophore-based
assay for human G protein-coupled receptors (Nuttall et al.,
1999).
When an in vitro purified target-based assay is feasible and a good relevant
mammalian cell-based secondary assay is available, there is generally no
strong rationale for a Drosophila approach. However, if there is difficulty in
making a usable mammalian cell-based assay and there is a well-established
fly-based functional readout for the target or a robust readout can be
engineered quickly, then a fly-based assay may be useful.
A Drosophila-based assay has the advantage of having a property against
‘assay drift’. In mammalian cell-based assays, due to multiple passages of cell
culture, there could be substantial loss of cellular response and changes of
assay statistics. This is most likely due to the combination effect of genetic
instability, manifested as an accumulation of aberrations in genetic material
that cannot be got rid of by mitosis, and genetic selection in cell cultures. In
this regard, a Drosophila whole-organism-based assay is far more stable. This
is because the fly culture is maintained by sexual reproduction – a process that
requires the stability of chromosome number and structure and involves
meiotic recombination. Most Drosophila mutant strains maintain their
original phenotypes even after many years, and any second-site genetic
modifiers that do accumulate can be removed by outcrossing to a wild-type
strain for a few generations. In cases where there is selection against a
phenotype produced using the multiple engineered components (transgenes)
necessary for an assay, these components can be maintained separately in two
different fly strains and brought together by mating in just one generation. For
example, by using the binary Gal4/UAS system (see Section 4.2; analytical
tools) the driver (Gal4 transgene) and the responder (UAS transgene) can be
maintained separately and then crossed to overexpress a target protein in the
progeny that produces a phenotype such as lethality. Compounds that inhibit
the activity of the target protein and/or pathway can be identified by virtue of
their ability to reverse the lethal phenotype.
The idea of using a disease pathway phenotype of Drosophila to look for
chemical modifiers, in very much the same logic as using a genetic screen for
genetic modifiers, is also worth exploring. In this case, the target in the
compound screen is the disease pathway and not a specific gene product. If the
assay phenotype is sufficiently validated, the chemical modifiers discovered
should have significant relevance. There are several unique features in this
approach. One obvious advantage of a chemical modifier screen is that it can
overcome the problem of genetic redundancy in a genetic screen. Second, in an